Multi-stages fiber amplifiers typically involve rare-earth doped optical fiber amplifiers optically coupled in series (i.e., cascaded fiber amplifiers). When coupled to a cascaded fiber amplifier system, an optical signal, such as, for example, a pulsed seed laser, can be amplified. A pulsed seed laser refers to a laser having an emission state that periodically changes from an on state (radiation emission) to an off state (no radiation emission). This periodic modulation of the seed laser may be accomplished by switching the power supply of the laser itself, or by using an external switch placed at its output. In the above-referenced design of cascaded fiber amplifiers, when the seed laser is in the on state, its power radiation is amplified at least by two fiber amplifiers. Those fiber amplifiers are usually designed such that the pulse energy is maximized at the device output.
A fiber amplifier is typically made of an optical fiber having a rare-earth doped core. A particular gain spectrum is associated to each type of rare-earth ions (also called dopant). In order to amplify light guided in the fiber core, rare-earth ions have to be in an excited state of energy. This energy is usually provided by the absorption of continuous wave laser pump light guided in the fiber core or cladding.
When the seed laser is in the off state, no signal light is incoming in the first amplifier doped core. Rare-earth ions in the amplifier cascade absorb the continuous wave pump light without amplifying the seed radiation. As a consequence, the potential gain of the amplifiers increases, and so does the energy stored in the fiber amplifiers. When the seed laser is switched on again, part of this stored energy is transferred to the traveling pulse by stimulated emission.
Rare-earth ions in an excited state spontaneously emit a photon within an average time span called the fluorescence lifetime. Even when an amplifier is not seeded by the master oscillator, spontaneously emitted photons are guided in the amplifier core and are amplified by neighboring rare-earth ions still in the excited state. The resulting light guided in the fiber core is called amplified spontaneous emission (ASE). The immediate consequence of ASE is the depletion of available energy in the doped fiber. When many amplifiers are used in series, ASE emitted by one amplifier may act as a seed and furthers depletion of stored energy in the other amplifiers as well.
In U.S. Pat. No. 5,933,271, Waarts et al. propose a solution to reduce the energy depletion in the first amplifier from ASE generated in the second amplifier by using pulsed pumping and synchronization. However, this solution complicates the amplifier configuration compared to continuous wave pumping.